Square-Planar Nickel Bis(phosphinopyridyl) Complexes for Long-Lived Photocatalytic Hydrogen Evolution.

Phosphinopyridyl ligands are used to synthesize a class of Ni(II) bis(chelate) complexes, which have been comprehensively characterized in both solid and solution phases. The structures display a square-planar configuration within the primary coordination sphere, with axially positioned labile binding sites. Their electrochemical data reveal two redox couples during the reduction process, suggesting the possibility of accessing two-electron reduction states.

Mirror-plane disorder in a nickel chloride Schiff base complex: a suitable case study for crystallographic instruction.

The nickel chloride complex of the Schiff base N,N-propanediylbis(2,3-butanedione-2-imine-3-oxime), namely, chlorido(3,9-dimethylundeca-3,8-diene-2,10-dione 10-oxime 2-oximato-κN,N',N'',N''')nickel(II), [NiCl(CHNO)], at 100 K crystallizes in the orthorhombic space group Cmce. The structure exhibits mirror disorder of the main molecule that is not present in the bromide analogue. The relatively small number of unique reflections in the data set and the disorder imposed by the crystallographic mirror plane present a challenging educational case study.

The reactivity of molecular oxygen and reactive oxygen species with [FeFe] hydrogenase biomimetics: reversibility and the role of the second coordination sphere.

The development of oxygen-tolerant H2-evolving catalysts plays a vital role for a future H2 economy. For example, the [FeFe] hydrogenase enzymes are excellent catalyst for H2 evolution but rapidly become inactivated in the presence of O2. The mechanistic details of the enzyme's inactivation by molecular oxygen still remain unclear.

One-Pot Tandem Photoredox and Cross-Coupling Catalysis with a Single Palladium Carbodicarbene Complex.

The combination of conventional transition-metal-catalyzed coupling (2 e process) and photoredox catalysis (1 e process) has emerged as a powerful approach to catalyze difficult cross-coupling reactions under mild reaction conditions. Reported is a palladium carbodicarbene (CDC) complex that mediates both a Suzuki-Miyaura coupling and photoredox catalysis for C-N bond formation upon visible-light irradiation. These two catalytic pathways can be combined to promote both conventional transition-metal-catalyzed coupling and photoredox catalysis to mediate C-H arylation under ambient conditions with a single catalyst in an efficient one-pot process.

Alkane Oxidation: Methane Monooxygenases, Related Enzymes, and Their Biomimetics.

Methane monooxygenases (MMOs) mediate the facile conversion of methane into methanol in methanotrophic bacteria with high efficiency under ambient conditions. Because the selective oxidation of methane is extremely challenging, there is considerable interest in understanding how these enzymes carry out this difficult chemistry. The impetus of these efforts is to learn from the microbes to develop a biomimetic catalyst to accomplish the same chemical transformation.

Exploring the kinetic and thermodynamic aspects of four-electron electrochemical reactions: electrocatalysis of oxygen evolution by metal oxides and biological systems.

Finding fundamental and general mechanisms for electrochemical reactions, such as the oxygen evolution reaction (OER) from water and reduction of CO2, plays vital roles in developing the desired electrocatalysts for facilitating solar fuel production. Recently, density functional theory (DFT) calculations have shown that there is a universal scaling relation of adsorption energy between key intermediate species, HO(ad) and HOO(ad), on the surface of metal oxides as OER electrocatalysts. In this paper, a kinetic and thermodynamic model for the four-electron electrochemical reaction based on previous OER mechanisms proposed by DFT calculations is developed to further investigate the electrocatalytic properties over a wide range of metal oxides and photosystem II.

Investigations by Protein Film Electrochemistry of Alternative Reactions of Nickel-Containing Carbon Monoxide Dehydrogenase.

Protein film electrochemistry has been used to investigate reactions of highly active nickel-containing carbon monoxide dehydrogenases (CODHs). When attached to a pyrolytic graphite electrode, these enzymes behave as reversible electrocatalysts, displaying CO2 reduction or CO oxidation at minimal overpotential. The O2 sensitivity of CODH is suppressed by adding cyanide, a reversible inhibitor of CO oxidation, or by raising the electrode potential.

Investigations of the efficient electrocatalytic interconversions of carbon dioxide and carbon monoxide by nickel-containing carbon monoxide dehydrogenases.

Carbon monoxide dehydrogenases (CODH) play an important role in utilizing carbon monoxide (CO) or carbon dioxide (CO2) in the metabolism of some microorganisms. Two distinctly different types of CODH are distinguished by the elements constituting the active site. A Mo-Cu containing CODH is found in some aerobic organisms, whereas a Ni-Fe containing CODH (henceforth simply Ni-CODH) is found in some anaerobes.

How light-harvesting semiconductors can alter the bias of reversible electrocatalysts in favor of H2 production and CO2 reduction.

The most efficient catalysts for solar fuel production should operate close to reversible potentials, yet possess a bias for the fuel-forming direction. Protein film electrochemical studies of Ni-containing carbon monoxide dehydrogenase and [NiFeSe]-hydrogenase, each a reversible electrocatalyst, show that the electronic state of the electrode strongly biases the direction of electrocatalysis of CO2/CO and H(+)/H2 interconversions. Attached to graphite electrodes, these enzymes show high activities for both oxidation and reduction, but there is a marked shift in bias, in favor of CO2 or H(+) reduction, when the respective enzymes are attached instead to n-type semiconductor electrodes constructed from CdS and TiO2 nanoparticles.

Investigations of two bidirectional carbon monoxide dehydrogenases from Carboxydothermus hydrogenoformans by protein film electrochemistry.

Carbon monoxide dehydrogenases (CODHs) catalyse the reversible conversion between CO and CO2 . Several small molecules or ions are inhibitors and probes for different oxidation states of the unusual [Ni-4 Fe-4 S] cluster that forms the active site. The actions of these small probes on two enzymes-CODH ICh and CODH IICh -produced by Carboxydothermus hydrogenoformans have been studied by protein film voltammetry to compare their behaviour and to establish general characteristics.

A unified electrocatalytic description of the action of inhibitors of nickel carbon monoxide dehydrogenase.

Several small molecules and ions, notably carbon monoxide, cyanide, cyanate, and hydrogen sulfide, are potent inhibitors of Ni-containing carbon monoxide dehydrogenases (Ni-CODH) that catalyze very rapid, efficient redox interconversions of CO(2) and CO. Protein film electrochemistry, which probes the dependence of steady-state catalytic rate over a wide potential range, reveals how these inhibitors target particular oxidation levels of Ni-CODH relating to intermediates (C(ox), C(red1), and C(red2)) that have been established for the active site. The following properties are thus established: (1) CO suppresses CO(2) reduction (CO is a product inhibitor), but its binding affinity decreases as the potential becomes more negative.

Bacteriohemerythrin bolsters the activity of the particulate methane monooxygenase (pMMO) in Methylococcus capsulatus (Bath).

Recently, a native bacteriohemerythrin (McHr) has been identified in Methylococcus capsulatus (Bath). Both the particulate methane monooxygenase (pMMO) and McHr are over-expressed in cells of this bacterium when this strain of methanotroph is cultured and grown under high copper to biomass conditions. It has been suggested that the role of the McHr is to provide a shuttle to transport dioxygen from the cytoplasm of the cell to the intra-cytoplasmic membranes for consumption by the pMMO.

Structural insights into the enzyme catalysis from comparison of three forms of dissimilatory sulphite reductase from Desulfovibrio gigas.

The crystal structures of two active forms of dissimilatory sulphite reductase (Dsr) from Desulfovibrio gigas, Dsr-I and Dsr-II, are compared at 1.76 and 2.05 Å resolution respectively.

Isolation, purification and characterization of hemerythrin from Methylococcus capsulatus (Bath).

Earlier work from our laboratory has indicated that a hemerythrin-like protein was over-produced together with the particulate methane monooxygenase (pMMO) when Methylococcus capsulatus (Bath) was grown under high copper concentrations. A homologue of hemerythrin had not previously been found in any prokaryote. To confirm its identity as a hemerythrin, we have isolated and purified this protein by ion-exchange, gel-filtration and hydrophobic interaction chromatography, and characterized it by mass spectrometry, UV-visible, CD, EPR and resonance Raman spectroscopy.

The C-terminal aqueous-exposed domain of the 45 kDa subunit of the particulate methane monooxygenase in Methylococcus capsulatus (Bath) is a Cu(I) sponge.

The crystal structure of the particulate methane monooxygenase (pMMO) from Methylococcus capsulatus (Bath) has been reported recently [Lieberman, R. L., and Rosenzweig, A.